1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotating force of a motor.
2. Description of the Prior Art
In the past, a conventional power steering system of this type has hitherto been known in which a motor is supplied with electric power from a battery so as to generate power-assisting force which is transmitted through a speed-reduction gear to a steering shaft by means of a gearing or belting so as to assist the steering operation of the steering wheel induced by an operator.
In such a conventional power steering system, however, there arise the following problem. Namely, if the battery is suddenly exhausted or has discharged below a certain level during travel of the vehicle for some reason, the motor can not be supplied with sufficient electric power so that the assisting power would suddenly be lost, requiring heavier steering effort from the operator and, in an extreme case, causing panic on the part of the operator. Moreover, in such a case, the motor, being connected with the steerable road wheels through the speed-reduction gear, also becomes part of the steering load thereby further increasing the steering effort required by the operator.